A difference from conventional user equipment (UE) applications is that many machine to machine (M2M) applications only receive and send small data. The small data usually is only several or tens of bytes. For example, M2M devices of a water meter and a power meter only need to exchange some small data with an application server. If a user plane bearer is established to transmit only the small data, network resources are wasted. The third Generation Partnership Project (3GPP) researches how to efficiently transmit small data on a premise that network resources are saved. One method is transmitting small data by using a non-access stratum (NAS) message of a control plane. The method avoids signaling overheads for establishing a user plane and is an effective small data transmission method.
To save power, some power saving technologies such as a power saving mode (PSM) and extended discontinuous reception (eDRX) are used in an M2M device. When a power saving function is enabled on the M2M device, the M2M device is temporarily in an unreachable state, and downlink data cannot be sent to the M2M device. For an M2M device using a power saving function, how to effectively transmit downlink small data is an issue that needs to be resolved.
The foregoing method for transmitting small data using NAS in the prior art is performed only when UE is reachable, and how to transmit downlink small data when UE uses a power saving technology and is temporarily in an unreachable state is not considered. When the UE uses a power saving technology and is in an unreachable state, the UE cannot receive downlink small data, thereby causing loss of the downlink small data.